WX GUI Scope Sink does not show up


I have just started using GNURadio and I was trying to do really simple
stuff with GNURadio Companion just to get a feel of how it works. But I
ran int some troubles doing the simplest stuff possible.
I tried just connecting a sinusoid source to a throttle and then to a WX
GUI Scope Sink just to see the signal generation.
But when I try to execute the flow graph, all I get is:


No scope window show up. Am I doing something wrong or is there
something wrong with GRC?
I just downloaded it from apt.
Thanks in advance.